Drill rods are lengths of hollow pipe which typically have a male thread at one end and a female thread at the other end. Drill rods range in size up to about ten meters long and a plurality of drill rods are assembled in forming a drill string by engaging the male thread of an upper drill rod with the female thread of an adjacent lower drill rod or vice versa. The lower end of a drill string culminates in a cutting head which drills through the ground when rotated. In operation, the upper end of the drill string is rotated by a drive mechanism mounted on a drilling rig from which the drill string downwardly projects. A drill string may project vertically from a drill rig or at a desired angle to vertical. Drill strings are commonly many hundreds of meters long and can be thousands of meters long. Rotation of the cutting head by action of the drive mechanism therefore necessitates that drill rods are rigidly connected together.
The making (connecting) of drill rod joints is typically performed manually by an operator locating the appropriately threaded end of a drill rod to be added to the drill string into the mating thread of the uppermost drill rod in the drill string so far assembled. The drill rod to be added to the drill string is then rotated into threaded engagement with the adjacent drill rod below it using a pair of stillsons. The drill string which has been lengthened by connection of the drill rod is then rotated by the drive mechanism until it moves a further distance into the ground, with a further drill rod then connected and so on. The operator must exert considerable force on the stillsons to connect the drill rods sufficiently firmly. The breaking (disconnecting) of drill rods from a drill string (tripping the string) is essentially the reverse of the making procedure, except that the torque required to disconnect rods is generally even greater than that required to connect them, so several operators using stillsons one or more meters in length may be required. Connections between drill rods may tighten during use due to the torque applied to the drill string and/or to any heating or deformation of the drill rods which may occur in use.
The breaking of drill strings is therefore inherently a particularly dangerous operation, with injuries to operators an ever present possibility. For example, severe injury can result if the drill string drive mechanism is accidentally actuated whilst a pair of stillsons are attached to a drill rod, and it is not uncommon for operators exerting considerable effort in forcing stillsons to slip and fall. It is therefore desirable to provide an apparatus which facilitates the disconnection of two drill rods.